The present invention relates to a method for the manufacture of seamless metal pipe by piercing a polygonal billet in a press roll piercing mill.
Among conventional methods for piercing a billet to form a seamless metal pipe (for example, seamless steel pipe), the press roll piercing method and the cross roll piercing method are known, and recently, a review of such piercing methods has begun. The press roll piercing method is a method for manufacturing a hollow shell by inserting a round or polygonal billet into a container and forcing a mandrel against the billet from one end of the container to force the thus guided billet between the press rolls of the piercing mill and over a piercing plug. However, this method has lost its competitive position because of product quality and efficiency. The cross roll piercing method is the one most widely employed at present, and among the variations of this method, the inclined roll piercing method is the most popular method. However, because of the severe working of the billet in this method, a high quality of rolled round billet is required.